This invention relates to a method and apparatus for inspecting the profile of the inner wall of a conduit, such as a heat exchanger tube in a steam generator, by a probe body that includes eddy current sensing coils. More particularly, this invention relates to a method and apparatus wherein the number of eddy current sensing coils used for sensing the conduit is increased without a corresponding increase in coaxial cables by the use of a bank switch. Still more particularly, this invention relates to a combination of a solid state switching circuit located in an eddy current sensing probe which houses eddy current coils for switching between respective sets of coils to increase the number of coil sensors without correspondingly increasing the number of coaxial cables.
The in-service inspection of heat exchanger and steam generator tubing in nuclear power plants, as well as in other energy sources, often requires that special eddy current probes be used to detect flaws such as circumferential cracks in the tube wall which are not detected by the standard bobbin eddy current probe. Thus, probes for inspecting the inner walls of metallic conduits are known to the art. One type of such probe is an eddy current probe having eddy current coils located on a probe body which is inserted into the interior of a tube and translated rotationally along its longitudinal axis. The probe body accommodates differences in the radius of the internal tube walls by using spring fingers which flex in a radial direction. Such eddy current type probes are generally formed by an eddy current coil resiliently mounted in a probe head which engages the interior of the tube being inspected when the probe is rotated. The probe coil is electrically connected to a current generator which conducts an alternating current to the coil as it is moved. An impedance detecting circuit which may take the form of an inductive bridge is also connected across the leads of the coil.
In operation, the alternating current conducted through the coil excites it into generating a pulsating magnetic field whose magnitude and polarity changes in accordance with the frequency of the current. When the coil of the probe is positioned in the vicinity of an electrically conductive wall, the changing magnetic flux emanating from the coil induces eddy currents in a portion of the wall. The characteristics of the eddy currents produced are dependent in part upon the specific impedance of the portion of the wall that conducts the eddy current. Since flaws in the metal walls create regions of higher resistance at flaw locations, eddy current probes may thus be used to locate flaws by constantly monitoring the impedances of the sensing coils as the probe body is moved along a predetermined path along the internal walls of the tube.
Representative examples of such apparatuses including means for moving the probe within the tube are shown in U.S. Pat. No. 4,876,506 to Brown et al, and U.S. Pat. No. 4,856,337 to Metala et al.
One such probe proposed in the prior art uses an eight-coil array to provide the required sensitivity for detecting tube flaws in a steam generator. However, the system lacks an ability to examine 100% of the circumference of the steam generator tube because of the positioning requirements of the coil and the corresponding conduit arrangements. Thus, the spaces between the coils effectively provide undetected spaces and may allow a flaw to remain undetected and thus become a safety as well as commercial concern for the plant owner.
Other methods are used to inspect 100% of the tube circumference, but in general such methods are more complicated. For example, these methods include rotating eddy current and rotating ultrasonic probes. Both of these methods have disadvantages for requiring considerably more time than the eight coil array probe for acquisition and analysis of the data.
Accordingly, it is an overall problem in the art to increase the coverage of an eddy current array probe. While this problem might be solved in a number of ways including redesign of the coil, or increasing the number of coils, the latter approach generally requires that the number of coaxial cables in the probe also be increased. The coaxial cables in the existing probes presently constitute a bulky arrangement which limits equipment usage used to deliver the probe through the tubing.
In current arrangements, as is well known, the probe assembly which connects the probe head housing the coils to the eddy current tester must be wound on a reel of a probe pusher to be driven through a curved conduit and along the bends in the steam generator tube. Additional coaxial cables to accommodate more coils would not allow sufficient flexibility to permit the probe manage normal bends in this steam generator.
A prior proposal to be applied to an ordinary insertion type probe employed in eddy current flaw detecting of various heat transfer tubes suggested the use of a number of small size coils arranged about the circumference of the probe and utilized a time sharing arrangement from the coils with a switching and signal processing circuit to convert the information into parallel signals using two coils per segment and four sets of coils. That system contemplated the same number of cables as the number of coil segments, recognizing that an increase in the number of cables between the probe and the flaw detector reduces the cable flexibility. Accordingly, the switching element for multiplexing was mounted on the probe for converting the change in the impedance occurring in the sets of coils in the probe into the time sharing signals switched every half cycle of a clock set at, for example, 6 KHz. Such a circuit required complexities in the circuitry to perform the multiplexing operation.
Accordingly, it is an overall object of this invention to provide an eddy current probe having an enhanced number of coils divided into two banks of coils while utilizing a conventional coil conduit system for the use of a switching circuit for switching the cable from one bank of coils to another.
It is another object of this invention to provide a semiconductor switching circuit in the probe head which houses the eddy current coils to switch between two sets of coils.
It is another object of this invention to increase the number of coils in a probe head which houses the eddy current coils from a normal set, such as eight, to an enhanced set, such as sixteen, without the addition of any additional coaxial cables connecting the coils to the head.
These and other objects of this invention will become apparent from the written description of the invention which follows, taken in conjunction with the accompanying drawings.